[Illustration:  Fig. 7.]

[Illustration:  Fig. 8.—­Measuring Intensities of Light.]

The intensity of light is easily measured.  Let two lights of different brightness, as in Fig. 8, cast shadows on the same screen.  Arrange them as to distance so that both shadows shall be equally dark.  Let them fall side by side, and study them carefully.  Measure the respective distances.  Suppose one is twenty inches, the other forty.  Light varies as the square [Page 38] of the distance:  the square of 20 is 400, of 40 is 1600.  Divide 1600 by 400, and the result is that one light is four times as bright as the other.

[Illustration:  Fig. 9.—­Reflection and Diffusion of Light.]

Light can be handled, directed, and bent, as well as iron bars.  Darken a room and admit a beam of sunlight through a shutter, or a ray of lamp-light through the key-hole.  If there is dust in the room it will be observed that light goes in straight lines.  Because of this men are able to arrange houses and trees in rows, the hunter aims his rifle correctly, and the astronomer projects straight lines to infinity.  Take a hand-mirror, or better, a piece of glass coated on one side with black varnish, and you can send your ray anywhere.  By using two mirrors, or having an assistant and using several, you can cause a ray of light to turn as many corners as you please.  I once saw Mr. Tyndall send a ray into a glass jar filled with smoke (Fig. 9).  Admitting a slender ray through a small hole in a card over the mouth, one ray appeared; removing the cover, the whole jar was luminous; as the smoke disappeared in spots cavities of darkness appeared.  Turn the same ray into a tumbler of water, [Page 39] it becomes faintly visible; stir into it a teaspoonful of milk, then turn in the ray of sunlight, and it glows like a lamp, illuminating the whole room.  These experiments show how the straight rays of the sun are diffused in every direction over the earth.

Set a small light near one edge of a mirror; then, by putting the eye near the opposite edge, you see almost as many flames as you please from the multiplied reflections.  How can this be accounted for?

Into your beam of sunlight, admitted through a half-inch hole, put the mirror at an oblique angle; you can arrange it so as to throw half a dozen bright spots on the opposite wall.

[Illustration:  Fig. 10.—­Manifold Reflections.]

In Fig. 10 the sunbeam enters at A, and, striking the mirror m at a, is partly reflected to 1 on the wall, and partly enters the glass, passes through to the silvered back at B, and is totally reflected to b, where it again divides, some of it going to the wall at 2, and the rest, continuing to make the same reflections and divisions, causes spots 3, 4, 5, etc.  The brightest spot is at No.2, because the silvered glass at B is the best reflector and has the most light.

When the discovery of the moons of Mars was announced in 1877, it was also widely published that they could be seen by a mirror.  Of course this is impossible.  The point of light mistaken for the moon in this secondary reflection was caused by holding the mirror in an oblique position.